Let each target determines whether a machine instruction is dead. If true, that allow...

Let each target determines whether a machine instruction is dead. If true, that allows late codeine passes to delete it.

This is considered a workaround. The problem is some targets are not modeling side effects correctly. PPC is apparently one of those. This patch allows ppc llvm-gcc to bootstrap on Darwin. Once we find out which instruction definitions are wrong, we can remove the PPCInstrInfo workaround.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@76703 91177308-0d34-0410-b5e6-96231b3b80d8